Engines may use various devices to reduce release of regulated emission. For example, engines may use emission control devices that perform various catalytic reactions to reduce release of regulated emissions. However, these devices may be sensitive to temperature, and engine operation may be adjusted to maintain these devices at specific operating temperature. The issue with temperature generation in the engine can be particularly difficult in diesel combustion systems, where increased combustion efficiency results in relatively low exhaust temperature.
One approach to provide increase heating is described in U.S. '790. According to this approach, exhaust heating can be performed where some cylinders are in high load condition, and some cylinders are in an engine braking condition. While such operation may increase exhaust heat, the inventors herein have recognizes that it may be relatively fuel inefficient due to the engine braking operation. Further, the differential torque between the cylinders may produce unacceptable vibration as some cylinders are producing negative torque while some are producing positive torque.
In another approach, described in '098, different power is generated between different cylinder banks, where more heat is provided from the higher power cylinders. However, the inventors herein have recognized that such operation may still provide inefficient heat since the cylinders operating at the lower power level produce relatively low amounts of heat.
The above issues may be addressed via a method for controlling operation of a diesel engine having a first group of cylinders and a second group of cylinders. The method may include operating the first group of cylinders with a first intake and exhaust valve timing, where at least one of the intake and exhaust valve timing is adjusted to a different timing compared with the second group of cylinders. For example, one group may have a more retarded intake valve timing (opening and/or closing) and/or a more advanced exhaust valve timing (opening and/or closing). Further, the cylinders may produce a differential level of combustion torque.
In this way, by using intake retard or exhaust advance on cylinders with less torque, it is possible to have a torque differential and also increase heat from both cylinder groups. In other words, both cylinder groups are able to generate at least some heat, and one group is able to generate increased heat due to operating at increased torque (to balance reduced torque of other group), while the other group is able to generate increased heat due to adjusted valve timing. Further, heat generation can be further enhanced in the other group, for example, by operating with adjusting fuel injection timing, amount, or number of injections. An advantage of such operation is that the cylinders providing less torque can do so without necessarily requiring increased throttling via a throttle in the intake (thus avoiding potentially increased pumping work).